1. Field of the Invention
The present invention relates to an electron gun with a cold cathode for use in a microwave electron tube such as a traveling-wave tube or the like, and more particularly to an electron gun for an electron tube with a cold cathode, which has at least two electrodes extending from a surface of the cold electrode.
2. Description of the Related Art
There has been known mounting of a cold cathode in an electron-beam device represented by a traveling-wave tube. However, no previous instance of mounting a cold cathode with a focusing electrode in such an electronbeam device as disclosed in the present invention is found in the art.
First, various conventional examples of mounting a cold cathode with no focusing electrode will be described below. Then, device mounting manners which could generally be derived from the conventional processes in order to mount a cold cathode with a focusing electrode will be described below.
1. Japanese unexamined patent publication No. 129144/97 discloses a linear beam microwave tube. As shown in FIG. 1 of the accompanying drawings, the disclosed linear beam microwave tube has cathode tip 52 with cold cathode 51 disposed on a surface thereof. Cathode tip 52 is joined by silver paste to a joint of mount support 53 which is supported by a package. Mount support 53 and cathode tip 52 are joined such that cathode tip 52 has end 55 abutting against reference surface 54 of mount support 53 and an opposite end spaced slight gap 56 from mount support 53. Cold cathode 51 can thus be accurately positioned with respect to reference surface 54. Wehnelt electrode 57 for focusing electron beam 50 emitted from cold cathode 51 is installed as follows: after Wehnelt electrode 57 has been shaped to a desired configuration, it is secured by heat-pressing to a gate electrode of cathode tip 52 so that the center of an opening of Wehnelt electrode 57 is aligned with the central axis of cold cathode 51. In operation, a predetermined potential is supplied to the gate electrode through Wehnelt electrode 57.
2. Japanese unexamined patent publication No. 115453/97 reveals an electron gun with a cold cathode. As shown in FIG. 2 of the accompanying drawings, the disclosed electron gun has first cylindrical insulator 61 and metal conductor 62 extending through a central hole in first cylindrical insulator 61 in an axial direction of the electron gun. Cold cathode 63 is mounted on emitter electrode 60 disposed on a tip end of metal conductor 62. An emitter potential of cold cathode 63 is led out of a vacuum space through metal conductor 62. First cylindrical insulator 61 and second cylindrical metal sleeve 64 disposed and abutting around first cylindrical insulator 61 are held respectively against gate electrode cylindrical metal sleeve 66 concentrically disposed around a shank of the metal conductor 62 and second cylindrical insulator 67 through conductive layer 65, abutting around gate electrode cylindrical metal sleeve 66. An end of second cylindrical metal sleeve 64 remote from conductive layer 65 is connected to a gate electrode of cold cathode 63 by metal bonding wire 68. A gate potential is thus taken out of the gate electrode through metal bonding wire 68, second cylindrical metal sleeve 64, conductive layer 65, and gate electrode cylindrical metal sleeve 66.
3. FIG. 3 of the accompanying drawings shows another conventional arrangement. As shown in FIG. 3, cold cathode emitter device 71 is fitted in a complementary recess defined in emitter electrode support 72. Cold cathode emitter device 71 is pressed against Wehnelt electrode 75 under the bias of spring 74 which acts between emitter electrode support 72 and another support 73 which is fixed in place and by which emitter electrode support 72 is supported. A gate electrode of cold cathode emitter device 71 is electrically connected to Wehnelt electrode 75.
Device mounting processes which could generally be derived from the above conventional processes in order to electrically draw out two independent electrodes, i.e., a gate electrode and a focusing electrode which are set up on a cold cathode emission surface will be described below with reference to FIGS. 4(a), 4(b), and 4(c) of the accompanying drawings.
The device mounting processes may include a Wehnelt electrode pressing process, a brazing process, and a resiliently biased fixing process. FIGS. 4(a), 4(b), and 4(c) show the resiliently biased fixing process.
For taking the potentials of two electrodes from the cold cathode emission surface, it is appropriate to take one electrode potential via the Wehnelt electrode. Specifically, as shown in FIG. 4(a), an electrode connected to the Wehnelt electrode is of necessity a focusing electrode due to structural limitations imposed to a cold cathode with such a focusing electrode. This is because no feeder lines can be exposed between the Wehnelt electrode and an anode disposed in facing relation to the Wehnelt electrode since an axially symmetrical electron lens should be formed between the Wehnelt electrode and the anode.
A gate electrode is usually electrically connected by a bonding wire or tab for taking an electrode potential therefrom at an outer periphery of the focusing electrode according to an electronic device mounting process.
FIG. 4(a) shows the use of a bonding wire for taking out a gate electrode potential, and FIG. 4(b) shows the use of a tab for taking out a gate electrode potential. FIG. 4(c) shows at enlarged scale structural details in the vicinity of a cold cathode illustrated in FIGS. 4(a) and 4(b).
For taking a potential from the gate electrode with a bonding wire, it is necessary that the bonding wire have a looped shape in order to keep its strength enough, and hence a space is required to accommodate such a looped shape. Furthermore, for maintaining a desired dielectric strength between the bonding wire and the Wehnelt electrode, the Wehnelt electrode needs to be large upwardly and diametrically, with the result that the electron gun cannot be reduced in size.
For connecting the bonding wire or tab to the cold cathode, it is necessary to exert forces to bonding wire or tag. At this time, dust particles are liable to stick between the gate electrode and the emitter electrode, developing an insulation failure therebetween which tends to cause the cold cathode to fail to operate.
Moreover, if the tab is used to connect the gate electrode, then the electron gun necessarily becomes large in outer dimensions because the electron gun needs to have a structure strong enough to withstand the driving of the tab.